gpui: Rename rounded_md to rounded_sm (#26228)

This PR renames the `rounded_md` style method to `rounded_sm`.

Follow up to https://github.com/zed-industries/zed/pull/26221, which
freed up the `rounded_sm` name.

Release Notes:

- N/A

crates/title_bar/src/collab.rs

@@ -220,7 +220,7 @@ impl TitleBar {
                 // When the collaborator is not followed, still draw this wrapper div, but leave
                 // it transparent, so that it does not shift the layout when following.
                 .when_some(leader_selection_color, |div, color| {
-                    div.rounded_md().bg(color)
+                    div.rounded_sm().bg(color)
                 })
                 .child(
                     Facepile::empty()